A comet is a comet is an asteroid, study suggests

New research is challenging the long-held position that comets and asteroids are as different as lions and lemurs.

The latest analysis of material from comet 81P/Wild 2 suggests that some comets are very asteroidlike and, presumably, vice versa.

"This is a wake-up call that small bodies in the solar system don't necessarily come in two flavors," said researcher Hope Ishii, a physicist at the Lawrence Livermore National Laboratory. "Instead, it's more of a continuum."

The new research, published in today's issue of the journal Science, contains the latest results from NASA's Stardust spacecraft, which flew through the tail of the comet and landed in the Utah desert in 2004.

Stardust, managed by the Jet Propulsion Laboratory in La Canada Flintridge, was the first spacecraft to return to Earth carrying comet dust.

Its initial results, released in late 2006, showed what scientists at the time called a "zoo" of materials, some of which came from the inner solar system where asteroids originated. As research into the comet dust samples has continued, Ishii said, a picture is emerging of a body that not only looks asteroidlike but is missing markers from the outer solar system, the home of most comets.

"We went to a comet, got a sample and brought it home," said University of Washington astronomer Donald Brownlee, a principal investigator on the Stardust mission.

"We all expected the picture that emerged to be simple. It's not."

Conventional scientific theory has long held that asteroids were cooked by the sun's heat before winding up in the asteroid belt between Mars and Jupiter. Comets, it was thought, were never cooked at all. Instead, they were thought to contain the most primitive material in the solar system: dust from other stars and other ancient material, as well as the ice and gas that give comets their tails when their orbits take them close to the sun.

This assumption was bolstered by decades of studies of comet dust captured by high-altitude balloons and aircraft.

Wild 2 tells a different story. Among the compounds scooped up by Stardust are calcium aluminum inclusions. These are produced by some of the highest-heating processes in the solar system, Ishii said.

Meanwhile, the research team found that two primitive materials normally found in interplanetary dust particles -- glass with embedded metals, and sliverlike rock-forming minerals -- have not been found in the expected quantities.

"The material is a lot less primitive and more altered than materials we have gathered through high-altitude capture in our own stratosphere from a variety of comets," Ishii said.

One possibility, Ishii said, is that the capture process destroyed some of the evidence. When the spacecraft flew through the comet's tail, its racket-shaped capture grid was slammed by thousands of particles, some the size of BBs.

But that doesn't explain why material only from the inner solar system would survive.

There is no doubt that Wild 2 is a comet. It spent much of its existence beyond the so-called frost line of the asteroid belt, where water survives as ice, Ishii said. In 1974, it passed close enough to Jupiter that its orbit was altered, sending it closer to Earth.

The capture itself was a delicate maneuver. After its 1999 launch, Stardust made three giant loops around the sun before flying through the comet's tail in January 2004.

The question raised by the new results is what mechanism would transport some small bodies into the asteroid belt while others are kicked all the way out to the Kuiper Belt beyond Neptune.

One theory, Brownlee said, is that the magnetic force of the sun acts as a slingshot.

In analyzing the Stardust material, Ishii's team used Livermore's SuperSTEM, a scanning transmission electron microscope.

John P. Bradley, a coauthor with Ishii of the new research, said he wasn't disappointed by the results.

"I think this is science in action," he said. "It's really exciting because it's just not what we expected."